Two component adhesive system useful as structural adhesive formulations have been described in the patent literature as represented, for example, by U.S. Pat. Nos. 3,706,426; 3,812,003; 3,935,051; 4,097,442; 4,373,082 and 4,444,976. In general, these adhesives comprise a prepolymer compound such as a polyisocyanate and a curative component such as a polyol which are mixed shortly before use. A drawback of the two component adhesive system resides in the fact that the two components must be properly measured and thoroughly mixed just prior to application. Consequently, many in the adhesives industry have turned to one component adhesive systems such as the polyurethane adhesives described in U.S. Pat. Nos. 4,496,706 and 4,515,933.
It is quite common to include a catalyst in both one and two component polyurethane systems to initiate and accelerate the curing of the composition. Most curing catalyst, however, catalyze the reaction of isocyanate and hydroxy compounds not only at elevated temperature but also at room temperature. As a result, attempts have been made to provide stable one component polyurethane adhesive compositions by employing what is commonly referred to in the art as a "latent catalyst", that is, a catalyst which is substantially inactive at room temperature but active at moderately elevated temperatures. Unfortunately, such catalyzed one component polyurethane systems have provided less than satisfactory results in both bonding and stability properties. Normally, attempts to increase bonding strength often result in a loss in stability and vice versa. Moreover, no polyurethane structural adhesives are known which give fiber-tearing bonds to untreated i.e. uncleansed sheet molded compound (SMC). This latter bonding difficulty is attributable to impurities in the SMC, e.g. low molecular weight polyols, mold release agents, etc.) that migrate to the surface thereof and impede bonding unless removed.
It is an object of the invention, therefore, to provide a two component structural adhesive system characterized by a long pot life at room temperature and a very rapid cure at moderately high temperature.
Another object of the invention is to provide an improved two component polyurethane adhesive system which in addition to the aforementioned long pot life and rapid cure rate, possesses improved bonding properties.
Yet another object of the invention is to provide an improved adhesive for sheet molded compound (SMC) bonding which effects a satisfactory bond without treatment or cleansing of the SMC surface.
A further object of the invention is to provide a laminated assembly comprised of a sheet mold compound laminated to a fiber-glass reinforced thermoplastic substrate with the adhesive system of the invention.